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Segment Information - Real Estate Investments, at Cost of Company's Reportable Segments (Parenthetical) (Details) - a | 6 Months Ended |
--- | --- | ---
Jun. 30, 2018 | Dec. 31, 2017
3025 and 3001-3003 JFK Boulevard [Member] |  |
Segment Reporting Information [Line Items] |  |
Lease agreement term | 99 years |
Pennsylvania Suburbs [Member] | Assets Held-for-sale [Member] |  |
Segment Reporting Information [Line Items] |  |
Acreage of land | 2.7 |
Pennsylvania Suburbs [Member] | Assets Held-for-sale [Member] | Land Held For Development [Member] |  |
Segment Reporting Information [Line Items] |  |
Acreage of land | 2.7 |
Other [Member] | Assets Held-for-sale [Member] | Land Held For Development [Member] |  |
Segment Reporting Information [Line Items] |  |
Acreage of land |  | 13.1

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